Adjustable heating device for crucible-free zone melting a crystalline rod

ABSTRACT

Device for crucible-free zone melting a crystalline rod, particularly a semiconductor rod, includes two induction heating coils annularly surrounding the crystalline rod, and driving means for adjusting the position of the induction heating coils relative to one another in a direction transverse to the rod axis. Method of using the device includes relatively moving the induction heating coils away from one another transversely to the rod axis at the junction of a seed crystal with one end of the rod so as to fuse the seed crystal to the rod, relatively moving the induction heating coils on the one hand and the rod on the other hand along the axis of the rod and simultaneously relatively displacing the induction heating coils gradually toward one another in a direction transverse to the rod axis in conformity with an outwardly flaring conical portion of the rod extending from the seed crystal, until the induction heating coils substantially coaxially surround the main cylindrical portion of the rod.

United States Patent [72] Inventor Woligang Keller Pretzield, Germany [21 Appl. No. 724,528 [22] Filed Apr. 26, 11968 [45] Patented. Nov. 23, I971 [73] Assignee Siemens Aktiengesellschaft Berlin, Germany [32] Priority Apr. 29, 1967 [33] Germany [31 S 109660 a [54] ADJUSTABLE HEATING DEVICE FOR CRUCIBLE- FREE ZONE MELTING A CRYSTALLINE ROD 4 Claims, 4 Drawing Figs. [52] U.S. Cl 23/273 SP, 23/301 SP, 219/1079, 219/1043 [51] Int. Cl B0lj 17/10, H05b 5/18 [50] Field of Search 23/273, 301;219/10.79, 10.43 [56] References Cited UNITED STATES PATENTS 2,905,798 9/1959 Freutel 23/273 Primary Examiner-Norman Yudkoff Assistant ExaminerR. T. Foster Allorneys-Curt M. Avery, Arthur E. Wilfond, Herbert L.

Lerner and Daniel J. Tick ABSTRACT: Device for crucible-free zone melting a crystalline rod. particularly a semiconductor rod, includes two induction heating coils annularly surrounding the crystalline rod, and driving means for adjusting the position of the induction heating coils relative to one another in a direction transverse to the rod axis. Method of using the device includes relatively moving the induction heating coils away from one another transversely to the rod axis at the junction of a seed crystal with one end of the rod so as to fuse the seed crystal to the rod, relatively moving the inductionheating coils on the one hand and the rod on the other hand along the axis of the rod and simultaneously relatively displacing the induction heating coils gradually toward one another in a direction transverse to the rod axis in conformity with an outwardly flaring conical portion of the rod extending from the seed crystal, until the induction heating coils substantially coaxially surround the main cylindrical portion of the rod.

11 2 ADJUSTABLE HEATING DEVICE FOR CRUCKBLE-F'REE Qther features which are considered as characteristic for ZONE MELTIING A CRYSTALLINE ROD the invention are set forth 111 the appended claims My invention relates to device and method for crucible-free Although th invention rs illustrated and described herern as e floating-zone melting a crystalline rod, particularly a semicondevrce and method for crucible-free zone melting a crystalline ductor rod, wherein two induction heating coils annularly sur- 5 rod, particularly a semiconductor rod, it is nevertheless not inround the crys al ine dtended to be limited to the details shown, since various modifi- It is, of course, known to fuse a seed crystal to the end of a cations may be made therein without departing from the spirit semiconductor rod in a crucible-free floating-zone melting of the invention and within the scope and range of equivalence regard to the dislocation density thereof. After the seed crystal companying drawings, wherein:

has been fused to the semiconductor rod, a melting zone is FIG. 1 is a perspective view of a device according to the inpassed through the entire semiconductor rod. To fuse the seed vention having two induction heating coils adjustably discrystal and the crystalline rod and to subsequently zone-melt placeable relative to one another in a direction transverse to a tion heating coils have different dimensions, the inner diame- FIG. 3 is a plan view of the heating coils of the device in a attained between both induction heating COliS and the respeccylindrical portion of the crystalline rod is being zone-melted. 73

3O coil windings are firmly coupled to one another coaxially at a portion differing in diameter from that of that of the trically either in series or in parallel. For the use at hand, the

coupling when a conical transition zone of the crystalline rod 1 and 2 m the axial direction of their prvot 3 IS, for example between the seed crystal and the cylindrical portion of the rod about 1 mm is being melted or when a crystalline rod having a supply rod The induction heating coils 1 and 2 can be connected elecmethods of this general type. More particularly, it is an object which are generally tubular in form. The leads of the induction of my invention to provide such method and device which will heating coils l and 2 are identified by reference characters la provide good coupling between the two induction heating and lb, on the one hand, and 2a and 2b, on the other hand, coils when fusing a conical transition zone between a seed and are secured, for example by hard soldering, to bushings or crystal and the cylindrical portion of the semiconductor rod sleeves 4a and 4b, on the one hand, and 5a and 5b, on the being treated. other hand. The bushings 4a, 4b and 5a, 5b consist of material it is a further object of my invention to provide optimum use of relatively good electrical conductivity such as silver, for exof available high-frequency energy m all phases of the zoneample, and are supported on a common pivot 3 located outmelting process, that is when fusing a thin seed crystal to a side the induction heating coils l and 2. The pivot 3 is formed thick supply rod and, as a melting zone is berng passed through of insulating material such as ceramic or a heat-resistant crystal toathickerrod portion recrystallizingfrom the melting a drive mechanism 8 through 10. The common drive accordance with the invention, a device of the aforemengear 10. The components8to 10 of the gear train can consist troned type. wherein both induction heating coils are adof a nonferromagnetlc material such asglass, for example, ora ustably movable relative to one another by a driving heat-resistant plastic material, such as mechanism in a direction transverse to the axrs of the rod. polytetrafluoroethylene, known by the trade name Teflon, for

which includes relatively moving the two induction heating through the induction heating coils l and 2. The insulating and simultaneously relatively displacing the induction heating another, on the one hand, and 2a and 2b from one another. on I the rod axrs in conformity with an outwardly flanng conical drive shaft 11 serves for pivoting both induction heating COllS poruon of the rod extending from the seed crystal until both I and 2, the shaft 11 actuatlng the gear tram 8 through 10 by coils substantially coaxially surround the main cylindrical pormeans of a nonillustrated reversible motor.

tion of the rod. The aforedescribed device operates as follows:

When a seed crystal i2 is fused to a crystalline rod 13, as shown in FIG. 3, for example, both induction heating coils l and 2 are displaced toward one another about the common pivot 33, for example, by actuating the drive shaft 11 in a clockwise direction, FIG. ll so that the seed crystal 12 is firmly coupled with the portions of the windings of the induction heating coils l and 2 which form an oval'or somewhat lens-shaped interspace (FIG. 3). After the fusion and melting of the seed crystal 12, the induction heating coils l a 2 are held stationary while the crystalline rod 13 is displaced axially toward the induction heating coils l and 2. Thereby, for a gradual conical transition between the seed crystal l2 and the cylindrical portion of the crystalline rod 113, as shown in FIG. 2, both induction heating coils l and 2 are returned to an initial position in which they are coaxial to one another and to the rod 13 as shown in FIG. 4. This is efi'ected by actuating the drive shaft lll in the opposite rotary direction than aforedescribed, namely in the counterclockwise direction as viewed from the right-hand side of FIG. 1.

The device of my invention is marked particularly by the fact that the induction heating coils l and 2 need not by exchanged or replaced for different diameters of the seed crystal l2 and/or for a transition of varying diameter located between the seed crystal l2 and the cylindrical portion of the crystalline rod 13.

It is also essential that during the entire zone-melting operation, both induction heating coils l and 2 be energized by high-frequency electric current. Thereby, the growth conditions in the crystalline rod 13 are not varied erractically and, consequently, the danger of the formation of dislocations is considerably reduced.

It is obvious that details of the aforedescribed and illustrated embodiment can be varied without deviating from the basic scope and concept of the invention. Thus, for example, both induction heating coils 1 and 2 can be secured to separate pivots. The induction heating coils l and 2 can also be displaced in opposite directions to one another. The device of my invention is also suitable particularly for producing dislocation-free semiconductor rods, for example of silicon, having relatively large diameters. In a method carried out by such a device, after the seed crystal 12 has been fused to the crystalline rod 13, a bottleneck-shaped constriction of about 2 as viewed from the right-hand side of mm. thickness and 20 to 25 mm. length is produced by moving the nonillustrated end holders of the rod away from one another. Also, for producing this bottleneck-shaped constriction, the coupling between the induction heating coils l and 2, on the one hand, and this portion of the crystalline rod can be optimally adjusted.

As has previously been mentioned, the device of my invention can also be employed with good results for zone melting crystalline rods whose thickness varies over the length thereof. in such a case, both induction heating coils l and 2 can always be adjusted to the optimum coupling between crystalline rod and the induction heating coil by displacement of the coils relative to one another in a direction transverse to the axis of the crystalline rod. A short and stable melting zone is thereby obtained during the entire zone-melting operation.

I claim:

1. In a device for crucible-free zone melting a crystalline rod, a pair of induction heating coils annularly surrounding the crystalline rod, driving means for adjusting the position of the induction heating coils relative to one another in a direction transverse to the axis of the crystalline rod, said induction heating coils being pivotable about a common axis located outside of said coils and extending substantially parallel to the axes of said coils and including drive means operatively connected in common to said induction heating coils for pivoting said coils about said common pivot axis.

2. Device according to claim 1 including a pivot pin having an axis coextensive with said common pivot axis, a plurality of bushings coaxially mounted on said pivot pin and turnable thereabout, said induction heating coils having a pair of leads,

respectively, each of said leads being connected t o a respective one 0 said bushings, said drive means comprising a gear transmission operatively connected to said bushings for turning said bushings and the coils connected thereto about said pivot pin.

3. Device according to claim 2 wherein said gear transmission comprises a pair of bevel gear segments, secured respectively to one of the bushings connected to the respective induction heating coils, and a bevel gear meshing with both of said gear segments.

4. Device according to claim 3 wherein said drive means includes a driven shaft, said bevel gear being mounted on said shaft and being rotatable thereby.

i i t 

2. Device according to claim 1 including a pivot pin having an axis coextensive with said common pivot axis, a plurality of bushings coaxially mounted on said pivot pin and turnable thereabout, said induction heating coils having a pair of leads, respectively, each of said leads being connected to a respective one of said bushings, said drive means comprising a gear transmission operatively connected to said bushings for turning said bushings and the coils connected thereto about said pivot pin.
 3. Device according to claim 2 wherein said gear transmission comprises a pair of bevel gear segments, secured respectively to one of the bushings connected to the respective induction heating coils, and a bevel gear meshing with both of said gear segments.
 4. Device according to claim 3 wherein said drive means includes a driven shaft, said bevel gear being mounted on said shaft and being rotatable thereby. 